1. Field of the Invention
The present invention relates to a method and apparatus for printing a black-and-white image, and more particularly to a method and apparatus which converts color image information to black-and-white image information and then obtains a black-and-white printout, based on the converted black-and white image information.
2. Description of the Related Art
The reduction in prices of scanners and the advent of digital cameras, etc., have recently made it easy to generate image data with a wide variety of image readers. In addition, with the development and broad spread of office equipment, it has become possible to embed a photographic image into a manuscript, such as a document, which is constituted only of characters, tables, line drawings and the like simply represented only as black and white (two values), etc. Furthermore, in photogravure, etc., a net-pointed manuscript is often employed as a manuscript for printing.
Thus, in place of a manuscript simply represented only as black and white, various information has recently been transmitted by one manuscript. Because of this, many of the manuscripts become complicated because they contain black-and-white characters, tables, line drawings, etc., represented mainly by two values, and silver-salt photographs or netted-point images (not limited only to photographic portions) which can represent a halftone, and consequently, the construction of a manuscript to be printed has become increasingly complicated.
In the case where a black-and-white image is printed by a copying machine, printer, or stencil printer, etc., there is a need to obtain a black-and-white print, by (1) reading a manuscript, in which characters, photographs, netted points, etc., are present together, with a monochrome scanner, (2) obtaining a multi-level image signal carrying black-and-white image information sampled in the unit of a pixel in horizontal and vertical scanning directions, (3) binarizing this multi-level image signal, and (4) obtaining binary output, based on the binarized image data.
When binarizing a multi-level image signal, a simple binarizing method, in which binarization is performed with a single threshold value as a basis, is generally employed in a manuscript constituted of characters, line drawings, etc. For a manuscript consisting of a photograph with a halftone, a binarizing method, such as a pseudo halftone representation method and the like, is employed. As a typical example of this pseudo halftone representation method, a dither method, an error-diffusing method, etc., are known.
The above-mentioned simple binarizing method is a method where a multi-level image signal is scanned in horizontal and vertical scanning directions and each pixel constituted with the multi-level image signal is binarized with a preset threshold value as a basis. The dither method is a method in which pseudo level representation is performed on recording paper, by fluctuating a threshold value by the use of a particular pattern in order to generate a dither matrix, scanning a multi-level image signal of this dither matrix in horizontal and vertical scanning directions, and binarizing the gray level of each pixel with each threshold value as a basis. The error-diffusing method is a method in which pseudo level representation is performed on recording paper, by scanning a multi-level image signal in horizontal and vertical scanning directions, propagating binary errors occurring in the surrounding pixels of a target pixel to the target pixel at a preset rate, and binarizing the image signal of the relocated target pixel.
The simple binarizing method is a method effective for an image, in which the contrast of characters, line drawings, etc., should be enhanced, because the method has the effect of putting black; pixels together at a portion where a change in the gray-level gradient is sharp. However, for an image with a smooth change in a gray-level gradient and a weak contrast, such as a photograph, etc., this method is not suitable to binarize a photograph image signal, etc., as it damages image information having a smooth change in a gray-level gradient.
On the other hand, the binarizing method, employing a pseudo halftone process which is performed by a dither method or error-diffusing method, is a method effective for an image with a weak contrast such as a photograph, etc., since it gives a smooth pseudo representation to a region where a change in a gray-level gradient is small. However, in an image with strong contrast, which is required to concentrate black pixels at a portion where a change in a gray-level gradient is sharp, such as a character, a line drawing, etc., this method is not suitable to binarize a character image signal, etc., because it gives rise to a white pixel (black pixel except inside) at a character portion (which should originally be a black pixel), etc., also reduces character sharpness, and causes a fine character which have lost its shape.
Therefore, in the process of binarizing an image signal obtained by reading, if the print of a black-and-white manuscript where characters, photographs, netted points, etc., are present together is to be obtained, it is preferable that for a region, represented mainly by two values, which is constituted of characters, tables, line drawings and the like (hereinafter referred to as a character region), binarization be performed by the simple binarization method and it is preferable that for a region, which can represent a halftone, such as a silver-salt photograph region, a netted-point image region and the like (hereinafter referred to as a halftone region), binarization be performed by a dithering method or error-diffusing method. As described above, for one frame amount of read image signals corresponding to the amount of a manuscript sheet, a technique for accurately discriminating a character region and a halftone region and performing optimum binarization for each region becomes necessary in order to neatly print a manuscript where characters, photographs, line drawings, etc., are present together. In addition, in a silver-salt photograph and a photograph of a net-pointed manuscript, if binarization such as netted-point processing, etc., is performed on the net-pointed manuscript, the problem of easy occurrence of Moire fringes, etc., will arise. Therefore, it is not preferable to simply perform the same binarization on both a silver-salt photograph and a photograph of a net-pointed manuscript. From such a point, it also becomes necessary to accurately discriminate regions of a character, a photograph, and netted points from the read manuscript information and perform optimum binarization according to the region type of a manuscript image.
Note that in stencil printers, a binarizing process according to the type of output unit as well as the discrimination of regions in a manuscript is necessary, for the following reasons: as a dot gain is great, image destruction is liable to occur; and if the same binarizing process is performed, an image darker in its entirety than ordinary printers will be printed out; and therefore, if binarization is simply performed, for example, on a halftone region by the error-diffusing method, reproduction of a clear image cannot be performed.
On the other hand, at present, embedding color photograph images as well as black-and-white photograph images, as is generally known, is possible. Also, in photogravure, etc., color net-pointed manuscripts are sometimes employed as manuscripts for printing. Thus, the rate at which a color manuscript is employed in printing has become higher than that for a black-and-white manuscript.
In the case where a black-and-white image is printed by the use of a color manuscript, a black-and-white print can be obtained in a manner known in the prior art, by reading the color manuscript with a monochrome scanner, obtaining a multi-level image signal representing only the brightness (gray level) information in the color image corresponding to the black-and-white image information, binarizing this multi-level image signal, and obtaining binary output, based on the binarized image data.
However, in such a simple method, only the brightness information in a color image is extracted and the hue information and saturation information cannot be utilized in performing region discrimination or binarization. Because of this, a black character, for example, is discriminated as a character and printed properly, while a light red character is discriminated as a photograph, not as a character. As a result, a phenomenon of a printed character becoming blurred, etc., will arise. In this case, a more appropriate printout can be obtained, by adjusting the gain of a multi-level image signal, or parameters that are employed in performing region discrimination or binarization. However, although a light red character becomes appropriate, the other regions conversely become inappropriate. Thus, adjustments cannot always be made so that an appropriate printout is obtained for all regions.
There is a method in which only specific color information is detected by employing a color conversion filter on a monochrome scanner and color classification is performed by the use of representation such as netting, etc. This method, however, is not effective and can perform only graph classification.
There is another method in which a black-and-white print is obtained by reading a color manuscript with a color scanner; obtaining, for example, multi-level color image signals carrying red (R) information, green (G) information, and blue (B) information; converting the three multi-level image signals to a single multi-level image signal (luminance signal) representing monochrome image information by the use of a known equation of Y=0.3R+0.6G+0.1B; binarizing this converted multi-level image signal; and obtaining binary output, based on the binarized image data.
According to this method, at least each color information about R, G, and B can be reflected on monochrome-image information. In addition, color adjustments are made by adjusting, for example, the gain of a desired color in each of the multi-level image signals for R, G, and B, and the result can be reflected as gray adjustments on a monochrome print. Therefore, it is conceivable to make adjustments so that a light read character portion is properly printed without having influence on the other portions.
However, the RGB color space represented by each color information about R, G, and B is not necessarily coincident with human color sensation, and there is a problem that the gray adjustments as a result of adjustments to the gains of R, G, and B will not always be appropriate. For instance, it is not entirely impossible but is substantially difficult to adjust a gray state on a print, which represents a dark and dull flesh color, to a desired gray state such as a gray state representing a light and vivid flesh color, etc., by gain adjustments to R, G, and B.
For example, even if a color image signal read by a color scanner is converted to a monochrome signal by the above-mentioned equation, it will not represent a desired hue (change in a gray level) such as the skin of a human being, the blue of the sky, the green of trees and plants, etc., called memorized colors of persons.
There has been provided no simple method in which a color manuscript is converted to appropriate gray-level information (brightness information and gray information) and printed as a black-and-white image.